This invention relates to an apparatus and method for vehicle emissions control and more particularly, to an apparatus and method for controlling the temperature of the exhaust gas stream exiting the exhaust manifold and entering an underfloor catalytic converter containing a multi-functional catalyst, e.g., a three-way conversion catalyst and a nitrogen oxides (xe2x80x9cNOxxe2x80x9d) trap.
Conventional lean-burn engine control systems include an air/fuel controller that delivers fuel to the engine intake manifold proportional to measured air mass to maintain a desire air/fuel ratio, lean of stoichiometric. Emissions of nitrogen oxides (xe2x80x9cNOxxe2x80x9d) from lean-burn engines (described below) must be reduced in order to meet emission regulation standards. Conventional three-way conversion (xe2x80x9cTWCxe2x80x9d) automotive catalysts are suitable for abating NOx carbon monoxide (xe2x80x9cCOxe2x80x9d) and hydrocarbon (xe2x80x9cHCxe2x80x9d) pollutants in the exhaust of engines operated at or near stoichiometric air/fuel conditions. The precise proportion of air to fuel that results in stoichiometric conditions varies with the relative proportions of carbon and hydrogen in the fuel. An air-to-fuel (xe2x80x9cA/Fxe2x80x9d) ratio of 14.65:1 (weight of air to weight of fuel) is the stoichiometric ratio corresponding to the combustion of a hydrocarbon fuel, such as gasoline, with an average formula CH1.88. The symbol xcex is thus used to represent the result of dividing a particular A/F ratio by the stoichiometric A/F ratio for a given fuel, so that xcex=1 is a stoichiometric mixture, xcex greater than 1 is a fuel-lean mixture and xcex less than 1 is a fuel-rich mixture.
Engines, especially gasoline-fueled engines to be used for passenger automobiles and the like, are now designed to operate under lean conditions as a fuel economy measure. Such engines are referred to as xe2x80x9clean-burn enginesxe2x80x9d. That is, the ratio of air to fuel in the combustion mixtures supplied to such engines is maintained considerably above the stoichiometric ratio (e.g., at an air-to-fuel weight ratio of 18:1) so that the resulting exhaust gases are xe2x80x9cleanxe2x80x9d, i.e., the exhaust gases are relatively high in oxygen content.
Although lean-burn engines provide enhanced fuel economy, they have the disadvantage that conventional TWC catalysts are not effective for reducing NOx emissions from such engines because of excessive oxygen in the exhaust. The prior art discloses attempts to overcome this problem by operating lean-burn engines with brief periods of fuel-rich operation. (Engines which operate in this fashion are sometimes referred to as xe2x80x9cpartial lean-burn enginesxe2x80x9d.)
The typical TWC catalyst provided in the exhaust passage as a xe2x80x9cclose-coupledxe2x80x9d catalytic converter does not convert the NOx produced when the engine is running lean, i.e., when xcex greater than 1. In order to reduce the NOx emission to the atmosphere, it is known to use an underfloor catalytic converter located downstream of the medium-coupled or close-coupled catalytic converter. xe2x80x9cClose-coupledxe2x80x9d catalytic converters are known in the prior art and are generally defined as located in or near the engine compartment, typically less than one foot, more typically less than six inches from, and preferably immediately adjacent to, i.e., attached directly to, the outlet of the exhaust manifold. xe2x80x9cUnderfloorxe2x80x9d catalytic converters are also known in the prior art and are located (downstream of any close-coupled catalysts)under the floor of the vehicle adjacent to or in combination with the vehicle""s muffler.
It is known to treat the exhaust of such engines with an underfloor catalytic converter containing a multi-functional catalyst, e.g., a TWC catalyst/NOx trap which stores NOx during periods of lean (oxygen-rich) operation, and releases the stored NOx during the rich (fuel-rich) periods of operation. A typical NOx trap utilizes alkali metal or alkaline earth metal oxides in combination with the precious metal catalyst component in order to store or occlude NOx under lean operating conditions. The mechanism for NOx storage is believed to involve the oxidation of NO to NO2 over the precious metal component of the TWC catalyst followed by the subsequent formation of a nitrate complex with the alkali metal or alkaline earth metal oxide. Under engine operation rich of stoichiometric (xcex less than 1), the nitrate complexes are thermodynamically unstable, and the stored NOx is released and catalytically reduced by the excess of CO, HCs and H2 in the exhaust. Periodically, the lean-burn engine is switched to a relatively rich air/fuel ratio to purge the NOx trap.
It is also known that exposure of the NOx trap to excessive temperatures, e.g. 750xc2x0 C. and higher, during the operation of the engine will result in a significant diminution of the capability of the NOx trap to absorb the NOx in the exhaust gas stream Therefore, it would be desirable to provide some means of insuring that the temperature of the exhaust gas stream entering the underfloor catalytic converter containing the NOx trap during the operation of the engine does not exceed the temperature at which the capability of the trap to absorb the NOx in the exhaust gas stream starts to fall off.
Lean-burn engines are designed for fuel economy. In such engines, operations alternate depending on speed and load. At the lean (xcex greater than 1) condition, the NOx trap in the underfloor catalytic converter absorbs NOx, then a fuel-rich (xcex less than 1) spike is applied which results in NOx desorption from the trap and catalytic reduction of the NOx to N2, then a lean condition occurs followed by a rich spike, etc. Rich conditions are required from time to time at higher speeds and loads in order to maintain the temperature of the exhaust gas flowing into the underfloor catalytic converter at a temperature below that which would result in deterioration of the NOx trap. At stoichiometric or richer than stoichiometric conditions, i.e. xcexxe2x89xa61, the multi-functional catalyst in the underfloor catalytic converter has the capability of reducing the NOx to N2 without absorption of the NOx by the trap.
Typically, the lean-burn engine is periodically switched to a relatively rich air/fuel ratio to purge the NOx trap. The NOx trap must be exposed to minimum threshold temperatures at specific engine speeds and loads before it will perform efficiently and accordingly, a minimum exhaust temperature must be established before a lean-burn mode of engine operation is established. There are also upper or maximum temperatures within certain speeds and loads, above which the trap will cease operating effectively for trapping NOx. At such higher temperatures with specific speeds and loads, the engine operation will switch from lean to stoichiometric (or rich) conditions. The catalyst in the underfloor catalytic converter will act as a TWC catalyst such that NOx, CO and HCs are effectively removed. Since fuel economy is significantly improved by operating the engine at lean conditions, an apparatus and method are required for sensing the upper temperature limit of the lean operation, thus permitting the temperature of the exhaust gas stream entering the NOx trap to be lowered, thereby allowing the lean operation of the engine to be extended to cover high speeds and loads which would otherwise be required to occur at stoichiometric conditions. Such extension of the lean operation of the engine would result in dramatically improved fuel savings, while concurrently lowering the formation of the greenhouse gas CO2.
It is also known that at certain higher speeds and loads, the temperature of the exhaust gas stream entering the NOx trap may exceed the temperatures at which the trap starts to deteriorate. At such extreme conditions of speed and load, the exhaust gas temperature is usually lowered by using fuel enrichment (xcex less than 1) in order to prevent deterioration of the trap. This practice has a severe adverse impact on fuel economy and also defeats the purpose of fuel savings associated with the lean burn engines. Accordingly, an apparatus and method are required for sensing such upper temperature limits in order to thereby lower the temperature of the exhaust gas stream entering the trap to a temperature below that which is known to cause deterioration of the trap, i.e., the temperature is thereby lowered to about 750xc2x0 C. or lower.
It is further known that over time, NOx breakthrough occurs when the trap becomes saturated with NOx and/or sulfur oxides (xe2x80x9cSOxxe2x80x9d) and must be purged in order to be able to continue to effectively trap the NOx. In order to purge the SOx, the NOx trap must be exposed to an exhaust gas stream having a temperature in the range of about 600xc2x0 C. to about 750xc2x0 C. Since the temperature and time required for purging SOx from the trap is close to that at which the trap suffers degradation, an apparatus and method is required for sensing NOx breakthrough and permitting exposure of the trap to the higher temperature exhaust stream during purging to remove SOx.
It is an object of the present invention to provide an apparatus and a method for controlling the temperature of the exhaust gas entering the underfloor catalytic converter containing a TWC catalyst/NOx trap such that the NOx trap will not be exposed to excessively high temperatures thereby causing a deterioration of the NOx trap during lean conditions of engine operation.
It is a further object of the invention to cause the lean-burn mode of operation of the engine to be extend cover higher engine operation temperatures (i.e., higher speeds and loads) than would usually be possible. It is the common practice, at the temperatures where the NOx trap become ineffective, to switch the engine operation from lean to stoichiometric, i.e.,xcex greater than 1 to xcex=1. This is necessary in order to facilitate the removal of the gaseous pollutants by the TWC function of the catalyst in the underfloor catalytic converter. The extended lean-burn mode of engine operation thereby improved fuel economy.
It is an additional object of the invention to provide an apparatus and a method which, at engine conditions requiring highs speeds and loads, result in a cooling of the exhaust gas to thereby minute the necessity of using fuel enrichment which would otherwise be required to effect such cooling in order to minimize deterioration of the NOx trap. This additional object also results in an improvement in fuel economy.
It is yet a further object of the invention to provide an apparatus and a method for purging the NOx trap of NOx and/or SOx without concurrent deterioration of the trap.
These and other objects have been achieved by the invention which is described in detail below.
In accordance with the present invention, an underfloor catalytic converter containing a multi-functional catalyst, e.g., a TWC catalyst/NOx trap, is provided in fluid communication with the outlet of the exhaust manifold of the engine. A first temperature sensor is provided in the exhaust outlet and a second temperature sensor is provided in the catalytic converter; (preferably near the inlet of the converter). A NOx sensor is located at the outlet of the converter in order to sensor NOx breakthrough, i.e., determine when the trap must be purged to remove NOx and/or SOx.
The first and second temperature sensors and the NOx sensor transmit their readings to a controller which transmits signals to proportional valve means located in the exhaust outlet downstream of the first temperature sensor. In response to the signals received from the controller, the proportional valve divides the exhaust gas stream from the engine exhaust manifold into two streams, one of which is sent to a first conduit and the other is sent through a second conduit into a heat exchanger. Any stream exiting the heat exchanger is then blended with the non-heat exchanged stream in the first conduit downstream of the proportional valve means.
The amount of the exhaust gas stream in the first conduit will vary from that in the second conduit such that (a) the temperature of the exhaust gas stream entering the catalytic converter is maintained in the range of about 300 to about 600xc2x0 C., preferably 350 to 550xc2x0 C., during the lean mode operation of the engine, and (b) the temperature of the exhaust gas stream entering the catalytic converter is periodically allowed to rise above about 600xc2x0 C. to the extent necessary to purge the NOx trap of SOx, i.e., when purging of the trap is required, little or none of the exhaust steam received from the exhaust manifold is sent to the heat exchanger.